In general, a GaN-based nitride semiconductor is being applied to an optical device of a blue/green Light Emitting Diode (LED), and an electronic device being a high-speed switching and high power device such as MESFET and HEMT. Specifically, the blue/green LED is under mass production, and its global sale is being exponentially increased.
Such a GaN-based nitride semiconductor light emitting diode is mainly grown on a sapphire substrate or a SiC substrate. Next, a thin film of polycrystalline AlyGa1-yN is grown as a buffer layer on the sapphire substrate or the SiC substrate at a low growth temperature. After that, an undoped GaN layer, a silicon (Si)-doped N—GaN layer, or an N—GaN layer having a combined structure thereof is formed on the buffer layer at a high temperature. A magnesium (Mg)-doped P—GaN layer is formed on the GaN layer to complete the nitride semiconductor light emitting diode. A light emitting layer (multi quantum well structured active layer) is sandwiched between the N—GaN layer and the P—GaN layer.
The P—GaN layer is formed by doping an atom of magnesium (Mg) in the growth of its crystal. The doped Mg atom should be substituted by gallium (Ga), thereby enabling the GaN layer to serve as a P—GaN layer, but is combined with a hydrogen gas released from a carrier gas and a source, to form a composition of Mg—H in a GaN crystalline layer and become material having a high resistance of 10 MΩ or so.
Accordingly, a subsequent activation process is required for disconnecting the composition of Mg—H and substituting the Mg atom with gallium (Ga) after the forming of a PN junction light emitting diode. However, the light emitting diode has a disadvantage in that an amount of carriers contributing to light emission in the activation process is 1017/cm3 or so, which is very lower than a Mg atomic concentration of more than 1019/cm3, thereby making it so difficult to form a resistant contact.
In order to improve this, a method for reducing a contact resistance using a very thin transmission resistant metallic material, thereby increasing an efficiency of current injection. However, the thin transmission resistant metal used to reduce the contact resistance generally has a light transmission of 75 to 80 percentages or so, and its remainder acts as a loss. Further, there is a limitation in improving the light output in the crystal growth itself of the nitride semiconductor, without improving a design of the light emitting diode and a crystallinity of the light emitting layer and the P—GaN layer, in order to increase an internal quantum efficiency.
Further, in the above structured light emitting diode, when a bias voltage is applied to the N—GaN layer and the P—GaN layer, electrons and holes are injected into N-type and P-type nitride semiconductor layers, and are recombined in the light emitting layer, thereby emitting light. Here, there is a drawback in that the light emitted from the light emitting diode is again partially reverse reflected inside at a boundary of the P—GaN layer and the contact layer, thereby decreasing the light output.